(1) Field of the Invention
The present invention relates to a metal vessel having a circumferential side seam. More particularly, the present invention relates to a bottle-shaped metal vessel which is highly improved in adhesiveness, sealing property and corrosion resistance of the seam. Furthermore, the present invention relates to an adhesive primer advantageously used for the formation of the seam of the above-mentioned metal vessel. More particularly, the present invention relates to an adhesive primer which has excellent bondability and adhesion to a copolyester type adhesive or metal substrate and also excellent adaptability to the cup-forming operation and the corrosion resistance of the seam.
(2) Description of the Prior Art
A bottle-shaped metal vessel having a circumferential side seam formed by lap-bonding circumferential open ends of cup-shaped upper and lower members obtained by draw forming or draw-ironing forming of a metal blank is advantageous in various points over a so-called can-shaped metal vessel.
As conventional packing metal vessels, there can be mentioned so-called three-piece cans in which can lids are double-seamed to the top and bottom of a can body having a side seam to form sealed portions, and so-called two-piece cans in which a can lid is double-seamed to an open end of a cup-shaped can body formed by drawing or draw-ironing of a metal blank.
However, these metal vessels having a double-seamed structure are insufficient in the pressure resistance of the sealed portion. Furthermore, these metal vessels involve a problem in connection with the saving of the material of the metal blank. More specifically, in the seam formed by double seaming, the material constituting the seam is first deformed by the load imposed on the seam, and leakage on from the seam or fracture of the seam is caused under a relatively small load by this deformation. In order to prevent this disadvantage, it is necessary that the thickness of the metal blank should considerably be increased. In the field of packing vessels, from the economical viewpoint and in order to reduce the weight of the vessel, it always is required to reduce the thickness of the metal blank. In the case where the thickness of the wall of the vessel barrel is reduced, buckling is readily caused by a load applied in the axial direction of the vessel at the double seaming step or the flanging or other preparing step.
In the case of a bottle-shaped metal vessel formed by lap-bonding open ends of cup-shaped upper and lower members, even if the thickness of the metal blank is extremely small, no deformation of the material constituting the seam is caused and the seam can resist up to a load corresponding to the shear strength of the seam irrespectively of the thickness of the metal blank. Furthermore, since the double seaming step is unnecessary, this metal vessel is advantageous in that the thickness of the side wall of the vessel can be reduced without a risk of buckling.
However, when a circumferential side seam is formed by lap-bonding open ends of cup-shaped lower and upper members, various limitations are imposed on adhesives used for lap bonding.
More specifically, although both the ends of a straight seam of a straight can body formed by lap bonding are mechanically secured by seaming with can lids, the above-mentioned circumferential side seam is not mechanically secured along the entire circumference at all, and the dimensional deformation of the seam per se is readily caused. Furthermore, since the diameter of the open end portion is going to change when the temperature changes, a stress is readily generated in the adhesive layer. Moreover, since the thickness of the open end portion constituting the seam is reduced in many cases, the seam easily undergoes deformation when an external force is applied. Therefore, it is required that the adhesive used should have a strong bonding force to the coating on the surface of the metal blank and that the adhesive layer should have high mechanical strength and dimension stability and the physical properties of the adhesive layer should be thermally stable and not be changed with the lapse of time.
Furthermore, the primer coating used should show high adhesion and bondability to both the metal blank and adhesive layer even under the above-mentioned severe conditions. Moreover, since the lower and upper members are formed by high draw processing or high draw-ironing processing, the adhesive primer and protecting coating layer should resist such processing and should have excellent corrosion resistance.